This invention relates to a function generator of time-dependent variable type which generates a time function waveshape which varies with time at a rate corresponding to time information applied externally for determining a time-dependent variable.
A function generator of time-dependent variable type which generates a time function waveshape varying with time is used as an envelope waveshape or control waveshape generator which generates waveshapes for imparting to a musical tone generated by an electronic musical instrument such amplitude envelopes or other control waveshapes as attack, sustain and decay. The envelope waveshape generator of this type is constructed to be set with an attack time (build-up time) as well as a decay time (build down time) of an envelope waveshape generated thereby by external information in any desired manner. More particularly, such envelope waveshape generator is constructed such that numerical value information for determining a time dependent variable regarding the attack time are repeatedly counted by a counter, and then repeatedly counting time information (numerical value information) regarding the decay time to form an envelope waveshape based on the count of the counter. The count of the counter varies at a rate corresponding to the time information so that the larger is the time information, the faster is the rate of changing the count and vice versa. Accordingly, the attack time and the decay time of the envelope waveshape which are formed based upon the count of the counter correspond to the time information. This type of an envelope waveshape generator is disclosed in U.S. Pat. No. 4,083,285 issued on Apr. 11, 1978.
Since the time information is generally expressed linearly by digital data having a limited number of bits, the length of the time set by the information is limited with the result that it is impossible to increase the length of the set time of the envelope waveshape. To solve this disadvantage, it has been proposed to increase the number of bits of the time information so as to increase the length of the set time. However, such proposal increases the size of the circuit.